Water-absorbing resin (super absorbent polymer or SAP) is a water-swellable, water-insoluble polymer gelling agent. Water-absorbing resin is widely used in various fields, including use in absorbent articles such as disposable diapers and sanitary napkins, agricultural and horticultural water retaining agents, industrial waterproofing agents and the like.
Many monomers and/or hydrophilic polymers are used as raw materials for water-absorbing resin. A polyacrylic acid (salt)-based water-absorbing resin containing acrylic acid and/or a salt thereof as a monomer is industrially produced in the largest amount, due to its water absorption performance.
Disposable diapers, which are one of the main applications of water-absorbing resin, have undergone advances in terms of performance. Along with these advances have come requirements for various functions (enhanced physical properties) of water-absorbing resin. Specifically, the water-absorbing resin is required to have, in addition to basic physical properties such as fluid retention capacity without pressure and fluid retention capacity under pressure, a variety of physical properties such as gel strength, water-soluble component, moisture content, water absorption speed, liquid permeability, particle size distribution, urine resistance, antibacterial property, damage resistance, powder fluidity, deodorant property, low coloring property, low dustiness, and low monomer residue.
Of the physical properties mentioned above, the liquid permeability is becoming a more important in view of an increase in the amount of water-absorbing resin used per disposable diaper. Proposed techniques for enhancing the liquid permeability include, for example, a technique of adding a spacer (Patent Literature 1) and a technique of using polyamine and either a polyvalent metal cation or polyvalent anion (Patent Literature 2). Proposed parameters relating to liquid permeability include, for example, pressurized void average radius index (Patent Literature 3) and wet porosity (Patent Literature 4).
Water absorption speed is also an important physical property of water-absorbing resin. Methods for increasing the water absorption speed include, for example, increasing the specific surface area of a water-absorbing resin. Specifically, proposed techniques include, for example, a technique of setting the particle diameter of a water-absorbing resin to be minute (Patent Literature 5) and a technique of foaming polymerization (Patent Literatures 6 and 7).
Note that liquid permeability and water absorption speed are physical properties having an inverse relationship. If one is improved, the other is impaired. In recent years, however, there have been proposed techniques for improving one without impairing the other, or for improving both. Specifically, proposed techniques include, for example, improved techniques for use in foaming polymerization (Patent Literatures 8 to 10) and improved techniques for use during gel-crushing (Patent Literature 11).